Volkswagen, the world’s largest automaker, is facing a new round of questions as US regulators say they have detected more cars produced by the automaker containing software that helped them cheat on emissions tests.

According to the EPA, the software in these engines increase emissions of nitrogen oxide (NOx) up to nine times the legal standard when the car is not attached to an emissions-measuring device. The EPA and the California Air Resources Board, which also has authority to regulate emissions, have launched investigations into Volkswagen’s actions.

“VW has once again failed its obligation to comply with the law that protects clean air for all Americans,” Cynthia Giles, assistant administrator for the EPA’s Office of Enforcement and Compliance Assurance said late Monday. “All companies should be playing by the same rules.”

Volkswagen, for its part, disputes the EPA's claims. The German carmaker said vehicles with the 3.0 liter diesel V6 engines "had a software function which had not been adequately described in the application process." The company vowed to work with US pollution regulators to "clarify" the situation.

“Volkswagen AG wishes to emphasize that no software has been installed in the 3-liter V6 diesel power units to alter emissions characteristics in a forbidden manner,” Volkswagen said in a statement.

But members of the House Energy and Commerce Committee, which is investigating the scandal, said it's time for Volkswagen to come clean, The Associated Press reports.

"The latest revelations raise the question, where does VW's road of deceit end?" said a joint statement by committee chairman Rep. Fred Upton (R) of Michigan and senior lawmakers from both parties.

The US arm of Porsche released a statement asserting its innocence. "We are surprised to learn this information. Until this notice, all of our information was that the Porsche Cayenne diesel is fully compliant," Porsche said.

In September, Volkswagen admitted that as many as 11 million diesel vehicles worldwide, including almost 500,000 in the US, from years 2009 to 2015, had the cheating software installed.

In another crack down on corporate environmental crime, four congressmen – Rep. Ted W. Lieu (D) of Calif.; Rep. Mark DeSaulnier (D) of Mass.; Rep. Peter Welch (D), of Vt.; and Rep. Matthew Cartwright (D) of Penn – on Friday asked the US Securities and Exchange Commission (SEC) to investigate ExxonMobil, one of the world’s largest oil companies, suggesting the company knew about climate change and failed to disclose the associated risks associated with their business, The Christian Science Monitor reports.

Within the past week, all three Democratic presidential candidates, former Secretary of State Hillary Clinton, Sen. Bernie Sanders (I) of Vermont, and former Maryland Governor Martin O’Malley, have called on the Department of Justice to investigate related accusations against Exxon.

The latest Volkswagen revelations affect about 10,000 diesel-powered sport-utility vehicles and largely sedans sold in the US since the 2014 model year. According to the AP, "Volkswagen faces fines of up to $37,500 per vehicle, which means up to $375 million could be added to penalties already projected in the billions of dollars."

Before the EPA's new findings were announced, a study in the journal Environmental Research Letters estimated that 59 people in the United States would die prematurely due to excess emissions caused by Volkswagen's cheating.